The ROS 2 design defines one Executor (instance of [rclcpp::executor::Executor](https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/executor.hpp)) per process, which is typically created either in a custom main function or by the launch system. The Executor coordinates the execution of all callbacks issued by these nodes by checking for available work (timers, services, messages, subscriptions, etc.) from the DDS queue and dispatching it to one or more threads, implemented in [SingleThreadedExecutor](https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/executors/single_threaded_executor.hpp) and [MultiThreadedExecutor](https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/executors/multi_threaded_executor.hpp), respectively.